A conventional operational amplifier typically includes an input stage and an output stage. The input stage receives differential input signals, and the output stage converts the differential input signals to a single-ended output signal. The input stage and the output stage are powered by a common power supply. For example, in a voltage measuring system, the conventional operational amplifier can be used to detect cell voltages of a multi-cell battery. If a common-mode input voltage level at an inverting input terminal and a non-inverting input terminal is higher than a first value, e.g., 5V, both the input stage and the output stage should be powered by a high-voltage power supply voltage, e.g., 18V, which is higher than the common-mode input voltage. Therefore, both the input stage and the output stage should use high-voltage transistors. A high-voltage transistor is a transistor with an operating voltage threshold larger than the high-voltage power supply voltage, e.g., 18V. Operation above the operating voltage threshold can degrade the reliability of or damage the transistor.
Typically, high-voltage transistors are large in size and have large parasitic parameters, so their use in an operational amplifier will increase the size of the operational amplifier and diminish its performance. Furthermore, it is difficult to shut off high-voltage transistors because their gate-source voltage threshold VGS is less than a second voltage level, e.g., 12V. Therefore, in conventional operational amplifiers, a level-shift circuit is employed to convert a low-voltage control signal to a high-voltage control signal, which is applied at the gates of the high-voltage transistors to shut them off. The level shift circuit further increases the size of operational amplifiers, and also increases their cost. Moreover, the level-shift circuit continues to function after the operational amplifier is shut down, which increases power consumption.